The invention relates to an optical system and components therefor for creating coloured fields of light.
In a continuously variable light beam coloration system, it is desired to change continuously the hue and saturation of the colour of a light beam in a fashion that renders the entire field to be evenly coloured for all degrees of light intensity. It is desired that the gamut of colours attainable by such a system cover as large an area of the 1931 CIE chromaticity diagram as possible. Furthermore it is desired that the intensity of the resulting colour field shall be independently controlled.
Systems that attempt such a result exist. It has been found that an acceptable range of colours can be obtained by combining three subtractive tristimulant colour filters in varying degrees. In high intensity light projectors such as are used for example in the entertainment and architectural lighting industries, the brightness of the light sources required is too high to use absorptive colour filters for colouring the light beam. So called dichroic filters are used instead which reflect the complementary colour of colour passed through the filter. Such filters display an extremely small absorption and are able to withstand the high ambient temperature and high intensity light throughput which are characteristic of such projectors. However such filters are expensive. A common configuration of such a subtractive tristimulant colour mixing system uses three filters, coloured cyan, magenta and yellow (CMY colour mixing). A further refinement may use a colour temperature correction filter (CTC) in addition which can be used to increase the gamut of available colours, but more particularly is used to vary the colour temperature of white light output. Any set of primary colours could be used to perform such colour mixing, however conventionally red, green, blue colour filters (RGB) are the only alternative to CMY actually used. Any reference in the following text to CMYC filters is equally applicable to any set of primary colour filters (plus CTC) and it is assumed that such alternatives are incorporated in any claims made.
Theoretically, any colour can be produced by combining the CMY filters to a varying degree. As an example, should a pale green colour be desired, a combination of cyan and yellow filters would be used to partially cover the output from a white light source. The degree to which the aperture is partially filled by a particular filter (and thus the degree of paleness of the colour attained) is the parameter known as the saturation. For example (theoretically) a fully saturated red would be achieved by the addition of fully saturated magenta in combination with fully saturated yellow. In practice due to the characteristics of dichroic filters, fully saturated colours are difficult to achieve by the addition of two subtractive colour filters. It is common in addition to the CMY filters to have a conventional colour wheel with red, green and blue filters mounted thereupon to achieve full saturation of these colours.
When mixing CMYC filters, it is essential that the level of saturation of a particular filter be evenly distributed across the field otherwise a mosaic or bands of colours rather than a mixture will be achieved.
The above filters are used in various light projection systems. Two principal methods are used to project light, each one having its respective advantages and disadvantages. Referring to FIG. 1, the condenser system provides the simplest system for evenness of lighting across the beam (flat field) and for projection purposes, however at the cost of efficiency and weight of the lens system. A substantial amount of the light output from the source is not transmitted via the projection lens, and this constitutes a loss in efficiency. Optics such as these are used when brightness is not as much a priority as image projection quality.
The most efficient (brightest for a particular light source) type of light directing system is the elliptical reflector system. This places the source at one focus of the ellipse and thus forms an image of the source at the other. It is common to place the CMYC filters as close to this second focus point as possible in order that the filters may be as small as possible. It will be noted from FIG. 2 that the incident light makes an angle xcfx86 with the filter xcfx86 naturally being dependent on from where in the source the light was emitted). It will also be noted that when the filter is placed at the focal point of the ellipse, the radial distribution of the light incident on the filter matches the radial distribution of the source. The intensity at the centre of the beam is thus significantly larger than on the periphery (this phenomenon is also termed the hotspot). Further useful observations about the two above optical systems that are pertinent to colour mixing systems, concern the characteristics of the source itself. For most applications, highly efficient discharge light sources are employed. These are brighter and give out less infra-red radiation then their more conventional halogen (filament) counterparts. Also, because light is produced at an arc gap, the source of light is smaller and thus behaves more like a point source which renders the optical system more efficient. However, the light that is incident on the colour filter has spectral and as well as intensity dependence on r, xcex8, and xcfx86 (see FIGS. 1 and 2). The effect is greatly reduced in condenser optics systems as xcfx86 is small in this case. If one is to a design a coloration system that is applicable to both optical systems certain conditions have to be fulfilled, and one comes to the following conclusion. If a certain percentage saturation of a particular filter is required, then ideally one needs to colour the same percentage for each of the components of incident light passing through the coloration system; moreover, that coloured portion needs to be evenly distributed over the range of each spherical componentxe2x80x94radial angle (xcex8), radial distance (r) and azimuth (xcfx86).
Having satisfied this condition, we will see that an even distribution of saturation is achieved across the entire field. This will then mean that the necessary conditions for varying intensity are necessarily also met. However, it is particularly desirable to provide a system which will permit the elliptical reflector system to function both in a projection (profile) environment and in a floodlighting (wash) environment.
The geometrical relationship shown in FIG. 3a relates to the use of an elliptical reflector system for projection purposes. The image to be projected (frequently of the gobo type which is a profiled cutout in an opaque material) has to be placed at the location where the cross section of light is smallest in order that maximum use is made of the light flux available. The ray path following the image has to held free of optical abberations, because a projection lens must create a high quality projected image at the intended image plane. This means that the filtering system for changing hue, saturation, and intensity may advantateously be placed in the ray path preceding the image. In order that the filtering system does not become unwieldy, the cross section of the light flux at the filtering system must not be too large. This in turn means the individual rays display a range of azimuth angle xcfx86 which is larger than in known condenser systems, and for this reason the improved filtering systems must be adopted.
The geometrical relationship shown in FIG. 3b relates to the use of an elliptical reflector system for floodlighting or wash purposes. In this type of use, the filters, including various effects not covered by the present application, have to placed at the location where the light flux is maximum. This location is then projected as a blurred image at a remote location by means of a short-focus projection lens incorporating a diffuser. In order that such a floodlight may be as small as possible, the elliptical reflector used creates a much larger range of azimuth angle xcfx86 than in the projection case, and this means a different adaptation of the filtering system according to the invention than in the case of image projection.
That the conditions are met is particularly important for systems using discharge sources for which intensity is necessarily varied by mechanical means. Indeed one aspect of the present invention is a mechanical dimmer system which utilises similar principles as the coloration system according to the invention.
Various methods have been tried to meet the conditions. One of the most commonly employed is the xe2x80x98fingerxe2x80x99 wheel consisting of a number of concentric wedges of dichroic filter material (see FIG. 4). By rotating the disc in an anticlockwise direction, the amount of the beam intercepted by the filter increases. The increase is very gradual, allowing for smooth changes in colour saturation. The wheel generally has a cut out (the glass removed entirely) for 0% saturation (no colourxe2x80x94known as open white) to remove any losses at the surface of the glass onto which the dichroic filter is adhered. The finger wheel has several substantial disadvantages. First of all, due to its large area, the cost of coating the glass wheel with the appropriate filter is quite high. Furthermore, it is expensive to cut a piece of glass in a customised shapexe2x80x94in this case circular with the cut out for open white. More fundamentally, the increase in saturation occurs from one side of the field only. Hence the distribution of coloration is asymmetric across the beam. Under certain circumstances, the fingers themselves can be seen entering the field from one side. This happens because of problems associated with etching the fingers on the wheel. It is very difficult to obtain portions of dichroic coating narrower than 1 mm using an etching process. The fingers are thus not sufficiently tapered at their extremes. For this reason, the transition between high saturation and full saturation can be very noticeable, as one side of the field is fully saturated and the other significantly less so. The difference is very obvious to the human eye as it is very sensitive to imbalances in the distribution of coloured light. However the dependency of the coloration of the beam as a function of xcfx86 is satisfied.
A variation of the finger wheel is a so-called raster wheel. This has a randomly distributed proportion of dichroic filter adhered which increases as the disc is rotated. Because the coating is randomly distributed, when the wheel is static the above conditions are met very satisfactorily. The same disadvantages of asymmetry apply as for the xe2x80x98fingerxe2x80x99 wheel, however. Moreover, the problems associated with etching are even more critical for a raster wheel. In addition there is the major disadvantage that the distribution, though random, does not change in a random mannerxe2x80x94it can be seen to move very perceptibly as the disc is rotated. A variation that alleviates the necessarily asymmetric nature of gradated wheels are colour flags. These are flags which gradually drop into the field by nature of being rotated about an axis. They come into the beam from diametrically opposite sides. To alleviate the obvious poor radial distribution, it has been known to give the edges of the flags a toothed structure.
One type of system (as described e.g in U.S. Pat. No. 5,426,576 and U.S. Pat. No. 5,969,868) achieves continuously variable hue and saturation of a light beam without using a combination of subtractive tristimulant colour filters. It uses continuously spectrally graded filters which have a continuously varying hue characteristic along a first axis and a continuously variable density (or saturation) along a second axis perpendicular to the first axis. Despite the effectiveness of the technique, the filters themselves are extremely expensive, costing some ten times more than the CMY systems described above.
Finally there are linearly translatable filters. Two toothed combs of filters are brought symmetrically into the field along a diametrical axis in a linear fashion (FIG. 5). The xe2x80x98fingersxe2x80x99 (or xe2x80x98teethxe2x80x99) on each comb interlace so that when the fingers are on the periphery (low saturation) as much light as possible is taken from the middle of the field (low r) as the edges (high r). It is an effective system, but in its simple form has one significant disadvantage which the present invention sets out to alleviate. It has been determined that while this type of filter works well in the environment of a condenser system, it does not work well with the elliptical reflector system, because of the much increased range of azimuth angle xcfx86.
Units for projection of profiled images and for floodlighting suffer from a tradeoff between the quality of the illuminated field and efficiency. According to the invention the optical components have been refined according to a general principle which enables the use of efficient elliptical reflectors without a reduction in quality for both purposes.
When the linearly translatable combs of interlacing fingers are illuminated with light that is not normal to the fingers, which which is the case for much of the light emitted from an elliptical reflector, light of the complementary colour to that desired suffers internal reflections between the fingers and passes through (see FIG. 17). Despite the coated side of the glass combs being put face to face, there is necessarily a gap (in the order of 1 mm) between combs which allows them to move over one another. This causes blotches of the undesired colour to be produced across the field (FIG. 16). One important aspect of the present invention seeks to rectify this colour unevenness. In the following, the expression xe2x80x9cwhitexe2x80x9d light may be used for the light entering a set of filters, irrespective of the filtering it may have been subjected to in preceding filters.